Carbohydrates or sugars, the most diverse class of biopolymers, are involved in many different biological processes. To be able to study these processes, well defined sugar structures are required. The synthesis of these sugar structures is at this moment far from ideal and therefore requires fundamental research, in particular towards the glycosylation reaction. In this reaction a positively charged oxocarbenium ion can be considered as the product forming intermediate, the cation is however commonly reasoned to lead to non-selective reactions and product mixtures. Chemical calculations on the oxocarbenium ion combined with model glycosylations proved the contrary, namely that these oxocarbenium ions are in fact selective. With these results, insight into the glycosylation mechanism is improved. The orientation of specific substituents on the sugar ring proved to have a profound influence on the stability of the oxocarbenium ion and thereby on the stereochemical outcome of glycosylations.